1. Field of the Invention
The present invention generally relates to oversampled or delta sigma (.DELTA..SIGMA.) analog-to-digital (A/D) converters and, more specifically, to a delta sigma A/D converter whose resolution tracks the noise level of the signal to be converted, thereby increasing the dynamic range of the conversion process.
2. General Description of the Prior Art
High resolution analog-to-digital (A/D) signal conversion can be achieved with lower resolution components through the use of oversampled interpolative (or delta sigma) modulation followed by digital low pass filtering and decimation. Oversampling refers to operation of the modulator at a rate many times greater than the Nyquist rate, whereas decimation refers to reduction of the clock rate by periodic deletion of samples.
Delta sigma modulators (sometimes referred to as sigma delta modulators) have been used in A/D converters for some time. Detailed general information can be obtained from the following technical articles which are hereby incorporated by reference:
1) "A Use of Limit Cycle Oscillators to Obtain Robust Analog to Digital Converters", J. C. Candy, IEEE Transactions on Communications, vol. COM-22, no. 3, March 1974, pp. 298-305 PA1 2) "Using Triangularly Weighted Interpolation to Get 13-Bit PCM from a Sigma-Delta Modulator", J. C. Candy et al., IEEE Transactions on Communications, vol. COM-24, no. 11, November 1976, pp. 1268-1275 PA1 3) "A Use of Double Integration in Sigma Delta Modulation", J. C. Candy, IEEE Transactions on Communications, vol. COM-33, no. 3, March 1985, pp 249-258
Broadly described, a delta sigma A/D converter uses an internal A/D converter of modest resolution and a complementary digital-to-analog (D/A) converter in a feedback loop. The feedback loop increases the accuracy of the A/D converter in a manner consistent with the high speed operation afforded by the internal A/D converter. In theory, any error in linearity or resolution caused by the D/A converter is effectively added to the input signal and appears at the output without attenuation.
Substantial effort has been expended in the field of oversampled A/D converter design to develop a delta sigma A/D converter with an increased dynamic range. Delta sigma A/D converters employ an analog integrator at the input to assure that the average of the digital output bits equals the average of the signal. This makes them inherently linear (assuming that the integrator is linear); however, a linear A/D converter wastes resolution in cases where the noise level is not uniform with signal amplitude. Attempts to match delta sigma A/D converters to applications that have non-uniform noise levels have used companding, but use of companding may make it necessary to restore linearity. This requires an exactly invertible companding characteristic.